The present invention relates to a spark generating apparatus with a pressure sensor.
An ignition coil for an internal combustion engine that is installed directly on an engine and that is directly coupled with spark plugs is known (e.g., a xe2x80x9cpencilxe2x80x9d coil). However, such conventional ignition coils and/or spark plugs do not generally incorporate a pressure sensor. A pressure sensor mounted on a spark plug is known as disclosed in U.S. Pat. No. 5,672,812 to Meyer.
Meyer discloses a magnetostrictive pressure sensor device attached to a spark plug shell. The disclosed approach, however, requires a magnetized spark plug shell. This increases cost, and subjects the resulting signal to noise due to environmental factors (e.g., magnetic and electrical noise in an automotive environment).
U.S. Pat. No. 6,122,971 to Wlodarczyk discloses use of a fiber optic for a pressure sensor integrated with a spark plug. U.S. Pat. No. 5,955,826 to Suzuki et al. disclose a spark plug with an opening in the threads to allow some combustion gases into a sealed chamber that houses a piezoelectric sensor for pressure sensing. This method has a disadvantage in that the opening can become clogged over time, impairing performance.
U.S. Pat. No. 6,119,667 to Boyer et al. disclose an integrated spark plug/ignition coil with a pressure sensor for an internal combustion engine. The sensor is disclosed as a magnetostrictive sensor, and is further disclosed as using a radially polarized biasing magnet and a sensing winding. As with the Meyer device, additional components such as a magnet are needed, which may increase cost and complexity. Thus, all of the foregoing are considered increased cost alternatives with drawbacks.
Ion sense methods are also known for detecting cylinder pressure; however, such methods are indirect approaches for determining pressure.
There is therefore a need for an improved spark generating apparatus with pressure sensing for an internal combustion engine that minimizes or eliminates one or more problems as set forth above.
An object of the present invention is to solve one or more of the problems as set forth in the Background. An advantage of an apparatus according to the invention is that it provides a high quality signal indicative of a pressure detected in a combustion chamber of an internal combustion engine. The high quality signal is generated at a comparatively higher level than certain other pressure sensors conventionally used. The present invention provides a high signal-to-noise ratio under all operating conditions. Moreover, if combustion pressures are to be measured by way of strains in the spark plug shell structure, the present invention provides a larger signal, and thus the improved signal-to-noise ratio mentioned above. The end result is a reduced cost, better quality product. Moreover, the invention avoids degradation over time due to a clogged opening, such as in Suzuki et al.
According to the invention, an apparatus is provided for initiating a spark that is suitable for installation in a cylinder of an internal combustion engine. The apparatus includes a central electrode having a main axis, an insulator, and a conductive shell. The central electrode has opposing first and second ends. The first end is configured for connection to a high voltage source. The insulator is outwardly of the central electrode. The conductive shell is generally disposed outwardly of the insulator, and includes a central bore. The shell also has an upper portion that extends radially, inwardly so as to cover and seal an upper part of the insulator. This is an upper seal, and it has an associated first spring rate. A lower seal is provided between the insulator and an inner shoulder feature of the shell facing the central bore. The lower seal has a second spring rate that is less than the first spring rate. The lower seal may be located proximate outer threads formed on the shell which are used to install and secure the apparatus to a cylinder head. The upper and lower seals are configured to prevent combustion gases from escaping from the cylinder, as known. Increased pressure in the cylinder will cause a deflection in the shell. A transducer is therefore affixed to the shell and is configured to produce a signal indicative of a pressure level in the cylinder.
In operation, the insulator is held by a preload between the upper and lower seals. When a pressure is applied to the bottom of the spark apparatus (i.e., such as when combustion occurs), static equilibrium is achieved by a change in the level of the loads of the upper and lower seals. The load in the upper seal will increase while the load in the lower seal will decrease. The stress (strain) in that portion of the shell between the seals will respond according to the change in the load in the upper seal. The change in the load in the seals will be divided based on the stiffness of respective load paths. The lower seal is located nearer to the threaded engagement to the cylinder head than is the upper seal. The threaded engagement is the location at which the additional load from the combustion is reacted into. Thus, the lower seal would generally have a stiffer load path to the reaction point. According to the invention, however, the lower seal is configured to have a reduced spring rate, relative to the upper seal, which effectively increases the stiffness of the load path of the upper seal relative to the lower seal. This arrangement dramatically improves the signal level obtained when measuring stress (and strains) near the upper seal to an increased deflection, thereby improving a signal-to-noise ratio of the resulting signal.
In one embodiment, the lower seal may comprise a Belleville washer that is arranged in preload so that it exhibits a small, zero, or even negative spring rate. This configuration enhances the amplification effect of the pressure signal described above.
In another embodiment, the invention may be applied to glow plugs for a diesel engine.